Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-06-22
2002-10-22
Kim, Ellen E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06470126
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a dispersion compensating optical fiber used in, for example, wavelength division multiplexing transmissions, and a wavelength division multiplexing transmission line using a dispersion compensating optical fiber.
BACKGROUND OF THE INVENTION
Single mode optical fibers having zero dispersion at a wavelength band of 1.3 &mgr;m have been laid in the world over as transmission networks for optical transmissions. Recently, the amount of transmission information has been remarkably increased in line with development of an information society. Further, in line with an increase in such information, a wavelength division multiplexing transmission (WDM transmission) has been widely accepted in the field of transmissions, wherein now an age of wavelength division multiplexing transmission has arrived. The wavelength division multiplexing transmission is a system in which the wavelength of optical transmission is not a single mode but wherein optical signals are transmitted with the wavelength divided into a plurality, and the system is an optical transmission system suitable for high bit rate and high rate transmissions.
However, in a case where a wavelength division multiplexing transmission is carried out using a wavelength band of 1.3 &mgr;m while using the existing single mode optical fiber having zero dispersion at a wavelength band of 1.3 &mgr;m, since the wavelength band (for example, 1.52 through 1.62 &mgr;m) of 1.55 &mgr;m which is a gain band of normal optical amplifiers is not coincident with the wavelength band of 1.3 &mgr;m, normal optical amplifiers cannot be used. Further, since the transmission at the wavelength band of 1.3 &mgr;m is large, there remains a problem which hinders long-haul optical transmissions. In order to avoid this problem, researches have been carried out, in which wavelength division multiplexing transmissions are executed at a wavelength band of 1.55 &mgr;m using the existing single mode optical fiber having zero dispersion at a wavelength band of 1.3 &mgr;m.
However, if an optical transmission is performed at a wavelength band of 1.55 &mgr;m using a single mode optical fiber having zero dispersion at the wavelength band of 1.3 &mgr;m, the dispersion of signals of respective wavelength division multiplexing wavelengths is increased as optical signals propagate in a transmission single mode optical fiber since the existing single mode optical fibers positive dispersion and positive dispersion slope at the wavelength band of 1.55 &mgr;m is increased. Therefore, it becomes difficult to separate signals at the receiving side, whereby the quality of optical transmission is worsened, and such a problem occurs, by which the reliability of optical transmission is lost.
Accordingly, recently, in order to solve these problems, a dispersion compensating optical fiber has been developed. The dispersion compensating optical fiber has negative dispersion. If the dispersion compensating optical fiber is connected to the receiving side of a single mode optical fiber, positive dispersion of optical signals transmitted through the single mode optical fiber is nullified by negative dispersion of the dispersion compensating optical fiber, wherein the optical signals are received in a state where the dispersion thereof at the receiving side becomes an almost zero dispersion. Thus, by connecting a dispersion compensating optical fiber to a single mode optical fiber, the respective wavelength division multiplexed optical signals can be separated at the receiving side, whereby a high bit rate and high rate optical transmission of high quality can be expected.
When using this type of dispersion compensating optical fiber, dispersion is compensated by connecting as short a dispersion compensating optical fiber as possible to the receiving side of a single mode optical fiber. To suit this usage, a dispersion compensating optical fiber developed previously is generally produced in the form of a module. As one of the examples, there is proposed a concept of an optical fiber suitable for a dispersion compensating optical fiber module disclosed in, for example, Japanese Laid-Open Patent Publication No. 11620 of 1994.
However, Japanese Laid-Open Patent Publication No. 11620 of 1994 does not provide any information regarding a profile of detailed refractive indexes of a dispersion compensating optical fiber. For this reason, it was impossible to specify the optimal refractive index profile of such dispersion compensating optical fibers as described above.
Further, in order to compensate dispersion by a short fiber length with a dispersion compensating optical fiber produced in the form of a module, it is necessary for a moduled dispersion compensating optical fiber to have high negative dispersion and high dispersion slope. If it is attempted that a dispersion compensating optical fiber has high negative dispersion and high dispersion slope, conditions of various types of parameters to define the refractive index profile of a dispersion compensating optical fiber are made severe, whereby it becomes very difficult to produce such dispersion compensating optical fibers. Moreover, if a dispersion compensating optical fiber is of such a refractive index profile structure as to have high negative dispersion and a high negative dispersion slope, a non-linear phenomenon is necessarily likely to occur, and the mode field diameter (MFD) of optical fibers is decreased. If the non-linear phenomenon occurs, the signal waveform may be distorted, which will cause another problem in achieving a high bit rate and a high rate in wavelength division multiplexing transmissions.
Further, as the mode field diameter of the optical fiber is decreased, still another problem arises, in which the non-linear phenomenon is likely to occur or the connection loss is increased when the dispersion compensating optical fiber is connected to a single mode optical fiber.
On the other hand, another optical transmission system is proposed, in which optical transmissions are carried out by using a dispersion shifted optical fiber in which the zero dispersion wavelength is shifted from a wavelength band of 1.3 &mgr;m to a wavelength band of 1.55 &mgr;m. By transmitting optical signals at a wavelength of 1.55 &mgr;m using a dispersion shifted optical fiber having zero dispersion at a wavelength band of 1.55 &mgr;m, it becomes possible to transmit signals free from any dispersion. However, in general, since a dispersion shifted optical fiber has a dispersion slope, no dispersion is produced for a zero dispersion wavelength at the wavelength band of 1.55 &mgr;m if a wavelength division multiplex transmission is carried out using signals at a wavelength band of 1.55 &mgr;m, but dispersion is produced with respect to signals in the vicinity of the wavelength band of 1.55 &mgr;m. Therefore, the optical transmission system in which a dispersion shifted optical fiber is used is not suitable for a high bit rate and high rate wavelength division multiplexed optical transmissions. Also, this type of dispersion shifted optical fiber has such a tendency, whereby a non-linear phenomenon is likely to occur, differing from a single mode optical fiber.
Therefore, the present inventor changed the conventional idea from a dispersion compensating optical fiber being made into a moduled optical fiber exclusive for dispersion compensation to an optical fiber which has both functions of dispersion compensation and action as a line which becomes a part of an optical transmission line. That is, in order to cause the optical compensating optical fiber to function as an optical transmission line, the length of a dispersion compensating optical fiber is made one-third or more the length of an optical fiber having a positive dispersion value at a wavelength band of 1.55 &mgr;m such as a single mode optical fiber; the dispersion compensating optical fiber is connected to an optical fiber having a positive dispersion value at a wavelength band of 1.55 &mgr;m; the dispersion compensating
Kim Ellen E.
LaCasse & Associates
The Furukawa Electric Co. Ltd.
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